(1) Field of the Invention
The present invention relates to a short arc high-pressure discharge lamp. The present invention also relates to a manufacturing method, a lighting method, and a lighting device for the lamp.
(2) Description of the Related Art
Recently, projection type display devices, such as a liquid crystal projector, have been actively developed. Such a projection type display device requires a high-intensity light source, which is close to a point light source. Therefore, high-pressure discharge lamps, such as a short arc extra-high pressure mercury lamp and a metal halide lamp, are generally used.
Originally, the construction of a tungsten electrode used in the short arc extra-high pressure mercury lamp and the metal halide lamp as the light source for the projection type display device was the same as that used in the conventional long arc high-pressure discharge lamp for a general purpose luminaire.
FIG. 1 shows a construction of an electrode 50 in the long arc high-pressure discharge lamp. As shown in this figure, the electrode 50 is composed of an electrode rod 51 made of tungsten and a coil 52 made up of a thin tungsten wire, with the coil 52 being set at a discharge side end of the electrode rod 51. This coil 52 has a function of dissipating heat and, as such, is set in order to prevent the electrode from overheating.
In the short arc lamp using the electrode 50, however, especially a temperature of the tip end 53 of the electrode further increases when compared to the long arc lamp, because a distance between electrodes is short. Thus, with thermal dissipation through the coil 52 only, it is impossible to keep the tungsten electrode material from melting and evaporating and to keep the tip end of the electrode from deforming and wearing. This gives rise to problems that deterioration of luminous flux from a lamp is caused and a lamp life is shortened.
Meanwhile, the recent trend for a short arc high-pressure discharge lamp for a projection type display device is to improve the luminance on the display screen. To achieve this by improving the efficiency for light utilization when combined with the reflecting mirror system, a lamp in a shorter arc type, in which the distance between electrodes is reduced from conventional 2.0-5.0 mm to 2.0 mm or less, has been developed. In this shorter arc type lamp, there is a new problem, specific to the shorter arc type lamp, that an xe2x80x9carc jumping phenomenonxe2x80x9d remarkably occurs. That is, as shown in FIG. 2, the electrode spot (the point where electrons are radiated out from the electrode as a cathode), which was originally formed in the vicinity of the tip ends of the electrodes 50 and 55 is not secured at one point, but moves at random with the passage of the lighting hours.
When this arc jumping phenomenon occurs, the discharge arc deviates from the optical axis of the lamp unit in which a reflecting mirror is incorporated. This causes the luminance on the display screen irradiated by the lamp unit to significantly fluctuate. Therefore, the market demands the development of a high-quality short arc high-pressure discharge lamp by which the luminance fluctuation on the display screen owing to the arc jumping phenomenon can be securely avoided.
The first object of the present invention is to provide a high-pressure discharge lamp whose lamp life is relatively long and resists the arc jumping phenomenon.
The second object of the invention is to provide a manufacturing method for the high-pressure discharge lamp.
The third object of the invention is to provide a lighting method which allows a high-pressure discharge lamp to have a long lamp life and to resist the arc jumping phenomenon.
The fourth object of the invention is to provide a lighting device which allows a high-pressure discharge lamp to have a long lamp life and to resist the arc jumping phenomenon.
The above first object is achieved by a high pressure discharge lamp made up of: an arc tube enclosing an arc chamber that contains a light-emitting substance and is hermetically sealed; and a first electrode and a second electrode that are spaced apart from each other, and each have an electrode rod coaxially extending into the arc chamber as a main body, and at least one of the first and second electrodes having a head integrally coupled with the electrode rod, on which a protrusion part is formed, wherein the head has a larger volume per unit length in the direction of the axis of the electrode rod than the electrode rod, and the protruding part is formed at a portion opposite to the other electrode.
The above second object is achieved by a method for manufacturing a high pressure discharge lamp in which a first and a second electrodes are provided so that tip ends of the electrodes face each other in an arc tube including the steps of: a head formation step for forming a head at an end of an electrode rod of at least one of the first and the second electrodes, wherein the head has a larger volume per unit length in the direction of the axis of the electrode rod than the electrode rod; a sealing step for sealing the first and the second electrodes so that the tip ends of the electrodes face each other at a predetermined interval; and a protruding part formation step for forming a protruding part on the head at a portion opposite to the tip end of the other electrode.
The above third object is achieved by (1) a lighting method for a high pressure discharge lamp in which a pair of electrodes that each have a head at an end of an electrode rod and a protruding part formed at an end of the head are provided so that the heads of the electrodes face each other in an arc tube, wherein each head is thicker than the electrode rod, including the steps of: (a) increasing a discharge arc current applied between the electrodes, in case that a monitor voltage between the electrodes becomes less than a predetermined voltage; and (b) decreasing the discharge arc current applied between the electrodes, in case that the monitor voltage becomes the predetermined voltage or more, in which these steps are alternately repeated, and (2) a lighting method for a high pressure discharge lamp in which a pair of electrodes that each have a head at an end of an electrode rod and a protruding part formed at an end of the head are provided so that the heads of the electrodes face each other in an arc tube, wherein each head is thicker than the electrode rod, including the steps of: (a) a first step for setting a frequency of an AC current applied between the electrodes at a first frequency when a monitor voltage between the electrodes rises to a first voltage; and (b) a second step for setting a frequency of the AC current at a second frequency, when the monitor voltage drops to a second voltage, the second voltage is lower than the first voltage, in which the first and the second steps are alternately repeated.
The above fourth object is achieved by (a) a lighting device for a high pressure discharge lamp in which a pair of electrodes that each have a head at an end of an electrode rod and a protruding part formed at an end of the head are provided so that the heads of the electrodes face each other in an arc tube, wherein each head is thicker than the electrode rod, made up of: a current generation means for generating a current which is applied between the pair of electrodes; a voltage detection means for detecting a voltage developed across the pair of electrodes; and a control means for controlling the current generation means and allowing the current generation means to adjust the current between the pair of electrodes according to a detected result by the voltage detection means, and (b) a lighting device for a high pressure discharge lamp in which a pair of electrodes that each have a head at an end of an electrode rod and a protruding part formed at an end of the head are provided so that the heads of the electrodes face each other in an arc tube, wherein each head is thicker than the electrode rod, made up of: a current generation means for generating an AC current which is applied between the pair of electrodes; a voltage detection means for detecting a voltage developed across the pair of electrodes; and a control means for controlling the current generation means and allowing the current generation means to adjust a frequency of the AC current between the pair of electrodes according to a detected result by the voltage detection means.